1. Field of the Invention
The present invention relates to a device for controlling steering force in a power steering apparatus wherein steering characteristics are varied by controlling assist force by means of an electronic control unit in accordance with conditions under which an automobile travels.
2. Discussion of the Prior Art
In the power steering apparatus of this kind, the travelling conditions of the automobile are generally detected on the basis of a car speed. The assist force is so controlled on the basis of the detected results that this force becomes large in a range of lower velocities but small in a range of higher velocities. For instance, as disclosed in Japanese Patent Laid-Open No. 9574/1984, a well-known power steering apparatus is arranged such that a plurality of control characteristics between the car speed and a rotational frequency of an oil pump are prepared, and one of these control characteristics is selected, depending on the conditions of streets of cities or towns and mountain roads. This type of apparatus is capable of varying the characteristics existing between manual steering torque and steering output torque by changing the assist force in accordance with not only a difference between the car speeds but also the difference between the road conditions.
However, there arise some problems in which the travelling condition of the automobile varies according to a feeling and a character of the driver in addition to the car speed and the road conditions, i.e., the travelling condition depends on the manner in which the driver drives the car aggressively or moderately, and the steering force suited to the operating condition determined by the driver can not be obtained in the prior art.